This application is based on applications Nos. 10-179089, 0-179090, 10-206788 and 11-93011 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a rewritable thermosensible recording material, a production method thereof and a rewritable thermosensible recording medium.
2. Description of Related Art
Conventionally, polymer cholesteric liquid crystal and low-molecular cholesteric liquid crystal are known as rewritable thermosensible recording materials. Polymer cholesteric liquid crystal is theoretically capable of changing its display color according to its temperature; such a change in color, however, requires a time on the order of a minute, which is an obstacle to practical use. As low molecular cholesteric liquid crystal, as disclosed in Advanced Materials, 1997. 9. (14), 1102-1104, only dicholesterol compounds are known. Such dicholesterol compounds are capable of changing their colors only within a narrow temperature range and cause chromatic unevenness, i.e., are poor in reproducibility.
An object of the present invention is to provide an organic low molecular rewritable thermosensible recording material and a production method thereof.
Another object of the present invention is to provide a rewritable thermosensible recording medium which is capable of displaying a full-color image at a high speed.
In order to attain the objects, a rewritable thermosensible recording material according to the present invention is shown by the following general chemical formula (A). 
The rewritable thermosensible recording material can be produced by condensing a derivative of dicarboxylic acid shown by the following general chemical formula (B) and cholesterol or by making halide dicarboxylic acid shown by the following general chemical formula (C) and cholesterol react to each other. It is possible to add a basic compound or an acidic compound as a catalyst. 
Examples of the basic compounds which are usable as the catalyst are hydroxide of alkaline metals, carbonate, hydrogencarbonate, alcholate. Also, organic bases such as quarternary ammonium salt, aliphatic amine, aromatic amine and dimethyl formacide can be used. It is preferred to use an organic base. Examples of the acidic compounds which are usable as the catalyst are various kinds of Lewis acid without any particular limitations. Typically, sulfuric acid, hydrochloric acid, nitric acid, boric acid etc. can be used.
For the reaction, the compounds (B), (C) and the catalyst are dissolved or dispersed in a suitable solvent, which can be selected from generally used various solvents. It is especially preferred to use a halogen solvent such as dichloromethane, dichloroethane, chloroform, trichloroethane, etc. or a cyclic ether solvent such as dioxane, tetrahydrofuran, etc.
The reaction may be carried out under atmospheric pressure in a temperature within a range from a room temperature to 150xc2x0 C. and may be carried out with pressure applied. After the reaction, the target compound can be obtained from the reactant by an ordinary method, for example, an ordinary separate purifying method such as filteration, concentration, crystallization, fractional distillation, recrystallization, etc. If a mixture is obtained after the reaction, the separate purification can be carried out by thin layer chromatography, liquid chromatography, column chromatography etc.
A rewritable thermosensible recording material according to the present invention is specifically shown by the above formula (A) and can be any of eleven compounds in which n is any integer from 8 to 18. The reaction to produce such a liquid crystal material is carried out for 30 minutes to ten hours in a temperature within a range from a room temperature to 150xc2x0 C. The completion of the reaction can be confirmed by use of thin layer chromatography, liquid chromatography, etc. After the reaction, the target liquid crystal compound can be obtained from the reactant by an ordinary separate purifying method such as filteration, concentration, crystallization, fractional distillation, recrystallization, etc. If a byproduct as well as the target liquid crystal compound are obtained simultaneously, the separate purifycation can be carried out by gas chromatography, liquid chromatography, column chromatography, a thin layer chromatography, etc.
A rewritable thermosensible recording medium according to the present invention has a recording layer containing the above-described rewritable thermosensible recording material. The recording layer contains the rewritable thermosensible recording material purely or as a mixture. This recording layer is located between a sheet-like base layer and a transparent protective layer. The recording layer is heated to a specified temperature by a heating device, for example, a thermal head or a hot stamp so as to show a specified color. Thereafter, the recording layer is heated to a specified temperature so that the color display can be erased, and thus, the recording layer is rewritable. Further, it is possible to add a photo-thermo converting agent such as an infrared absorbent to the recording layer or to a neighboring layer, which permits writing of an image by radiation of a laser.
Another rewritable thermosensible recording medium according to the present invention comprises a sheet-like base layer and a recording layer, and the recording layer contains at least two low molecular cholesteric liquid crystal compounds which exhibit a cholesteric phase in a temperature higher than a room temperature, in which the low molecular cholesteric liquid crystal compounds reflect light within a visible wavelength range according to the temperature, and are solidified while remaining in the reflective state by being cooled rapidly from the temperature.
In the recording medium, because the base layer is sheet-like and because at least two low molecular cholesteric liquid crystal compounds are used as the rewritable thermosensible recording material of the recording layer, it is possible to display a desired color at a high speed by heating the recording medium to a specified temperature according to the information to be recorded, and full-color display is also possible. Also, the displayed information can be erased by reheating of the recording medium. This recording medium can display a large number of colors without chromatic unevenness and can change its color within a wide temperature range, and the temperature control is easy.
Typical combinations of low molecular cholesteric liquid crystal compounds which can be used for the recording medium according to the present invention are shown by the following chemical formulas (D), (E), (F), (G), (H), (I) and (J). 
Such a combination may be a combination of low molecular cholesteric liquid crystal compounds of the same fundamental structure with different alkyl chain lengths and may be a combination of low molecular cholesteric liquid crystal compounds of different fundamental structures. Also, it is possible to combine three or more low molecular cholesteric liquid crystal compounds in consideration for the performance of the compounds. Besides the above compounds, various low molecular cholesteric liquid crystal compounds with cholesterol groups are usable.
It is preferred that at least one of the used low molecular cholesteric liquid crystal compounds has a molecular weight of 1000 to 1500. If the molecular weights of the low molecular cholesteric liquid crystal compounds are 1000 or less, the recording layer will be poor in memory performance. If the molecular weights of the low molecular cholesteric liquid crystal compounds are 1500 or more, the recording layer will be poor in responsibility to writing, and the transition temperature to the cholesteric phase of the liquid crystal will be too high.
Another rewritable thermosensible recording medium according to the present invention comprises a sheet-like base layer and a recording layer, and the recording layer contains a low-molecular cholesteric liquid crystal compound which exhibits a cholesteric phase in a temperature higher than a room temperature, in which the low molecular cholesteric liquid crystal compound reflects light within a visible wavelength range according to the temperature, and is solidified while remaining in the reflective state by being cooled rapidly from the temperature, and a nematic liquid crystal compound.
As the nematic liquid crystal compound, generally used ones, for example, liquid crystal cyanobiphenyl compounds, liquid crystal phenylcyclohexyle compounds, liquid crystal ester compounds, liquid crystal tolan compounds, liquid crystal pyrimidine compounds, liquid crystal bicyclohexyle compounds, liquid crystal phenyldioxane compounds and mixtures of these compounds can be used. It is possible to add a chiral agent which has a substituent with asymmetric carbon to the nematic liquid crystal compound. As the low-molecular cholesteric liquid crystal compound, for example, 10, 12-dicholesteril docosadiindioic acid can be used.